The Microstructure And Mechanical Property Of Lead-free Solder/copper Single Crystal Interface

In this paper, three typical lead-free solders were bonded to copper single crystal by reflow welding. The evolution of microstructure and the morphology at the three interfaces of lead-free solder/copper single crystal joints were investigated by isothermal aging experiment. Tensile and fatigue tests were carried out to investigate the tensile and fatigue properties of the soldering samples. The initiation and propagation of cracks were discussed in order to reveal the effect of aging on the interfacial deformation and fracture mode.The microstructure of Sn-4Ag/Cu and Sn-3 Cu/Cu interface is changed from singular Cu₆Sn₅ layer to duplex layers of Cu₆Sn₅ and Cu₃Sn phases after aging treatment; however, the microstructure of Sn-9Zn interface only consists of Cu₅Zn₈ layer before and after aging treatment. With increasing the aging time, the thickness of IMC layers all increases and the morphology of the IMC layers prefers to change from irregular type to planar type.The experimental results show that the mechanical properties of the soldering samples decrease with aging time. Sn-Ag soldering samples have the best mechanical property, and Sn-Zn samples show the worst. The thickness and the morphology of the IMC layers are the most important factors which determine the initiation and propagation of cracks. Cracks first occurred at the corner of solder joint. For the initial IMC interface with irregular shape, the cracks first initiated at the protuberance of the phase, and then propagated into the surrounding solder, exhibiting a typical feature of ductile fracture. When the irregular IMC grew into planar IMC layer, the cracks are easy to nucleate along the interface between IMC and solder. With further growth of IMC layer, the cracks mainly initiated and propagated within the thick IMC layer, leading to certain brittle fracture. Among the interfaces of IMC/solder, IMC/Cu substrate and IMC/IMC, the cracks are found to preferentially nucleate at the IMC/solder interface, but is difficult to form at IMC/IMC and IMC/Cu interfaces. Based on the results above, moreover, the fatigue cracking mechanism of vertical crack at the interface wass discussed.